Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan.
Plant Physiol. 2010 Mar;152(3):1529-43. doi: 10.1104/pp.110.153387. Epub 2010 Jan 22.
Abscisic acid (ABA) is postulated to be a ubiquitous hormone that plays a central role in seed development and responses to environmental stresses of vascular plants. However, in liverworts (Marchantiophyta), which represent the oldest extant lineage of land plants, the role of ABA has been least emphasized; thus, very little information is available on the molecular mechanisms underlying ABA responses. In this study, we isolated and characterized MpABI1, an ortholog of ABSCISIC ACID INSENSITIVE1 (ABI1), from the liverwort Marchantia polymorpha. The MpABI1 cDNA encoded a 568-amino acid protein consisting of the carboxy-terminal protein phosphatase 2C (PP2C) domain and a novel amino-terminal regulatory domain. The MpABI1 transcript was detected in the gametophyte, and its expression level was increased by exogenous ABA treatment in the gemma, whose growth was strongly inhibited by ABA. Experiments using green fluorescent protein fusion constructs indicated that MpABI1 was mainly localized in the nucleus and that its nuclear localization was directed by the amino-terminal domain. Transient overexpression of MpABI1 in M. polymorpha and Physcomitrella patens cells resulted in suppression of ABA-induced expression of the wheat Em promoter fused to the beta -glucuronidase gene. Transgenic P. patens expressing MpABI1 and its mutant construct, MpABI1-d2, lacking the amino-terminal domain, had reduced freezing and osmotic stress tolerance, and associated with reduced accumulation of ABA-induced late embryogenesis abundant-like boiling-soluble proteins. Furthermore, ABA-induced morphological changes leading to brood cells were not prominent in these transgenic plants. These results suggest that MpABI1 is a negative regulator of ABA signaling, providing unequivocal molecular evidence of PP2C-mediated ABA response mechanisms functioning in liverworts.
脱落酸（ABA）被认为是一种普遍存在的激素，在种子发育和植物对环境胁迫的反应中起着核心作用。然而，在代表陆地植物最古老谱系的苔藓植物（Marchantiophyta）中，ABA 的作用被强调得最少；因此，关于 ABA 反应的分子机制的信息很少。在这项研究中，我们从苔藓植物 Marchantia polymorpha 中分离并鉴定了 MpABI1，它是 ABSCISIC ACID INSENSITIVE1（ABI1）的同源物。MpABI1 cDNA 编码一个 568 个氨基酸的蛋白质，由羧基末端蛋白磷酸酶 2C（PP2C）结构域和一个新的氨基末端调节结构域组成。在配子体中检测到 MpABI1 转录物，其表达水平在外源 ABA 处理下增加，而 ABA 强烈抑制其生长。使用绿色荧光蛋白融合构建物的实验表明，MpABI1 主要定位于细胞核中，其核定位由氨基末端结构域决定。在 M. polymorpha 和 Physcomitrella patens 细胞中转瞬表达 MpABI1 导致与小麦 Em 启动子融合的β-葡萄糖醛酸酶基因的 ABA 诱导表达受到抑制。表达 MpABI1 和其突变构建体 MpABI1-d2 的转基因 P. patens 对冷冻和渗透胁迫的耐受性降低，与 ABA 诱导的晚期胚胎丰富样沸腾可溶性蛋白的积累减少有关。此外，这些转基因植物中不明显的 ABA 诱导的形态变化导致亲代细胞。这些结果表明，MpABI1 是 ABA 信号的负调节剂，为 PP2C 介导的 ABA 反应机制在苔藓植物中的作用提供了明确的分子证据。
